Iron oxide nanoparticles are attractive candidates for various in vivo applications because of their biocompatibility and high Curie temperature. Interest in iron oxide nanoparticles also extends to disciplines like bioengineering, magnetic resonance methodology and environmental science. However, gaining molecular-level insights into the surface chemistry and physical properties of iron oxide nanoparticles in liquid solutions (more generally any liquid-NP interface) is non-trivial and limited to a few analytical techniques. Here we introduce the use of X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) in conjunction with a liquid microjet to study the electronic, geometric and spatial distributions of hematite (Fe2O3) nanorods in liquid solutions of water and ethanol. XAS spectra were obtained at the Fe L3 edge via partial-electron-yield (PEY) detection based on resonant Auger-electron spectroscopy (LMM Auger transitions). We observe an increase in the t2g unoccupied density of states for hematite nanorods in aqueous solution compared with ethanol. Since PEY-XAS is largely ‘surface sensitive’ we interpret this increase to result from different electronic structures at the Fe2O3 nanorod-solvent interface. XPS of the Fe 3p orbital shows remarkably different spatial distributions of the nanorods at the air (vacuum) interfaces of the two different solvents. Fe2O3 appears to be at (or very near) the air interface in water, whereas it is completely excluded from the air interface in ethanol. The molecular origins of the different spatial distributions will be discussed. These results pave the way for studying the electronic structure and reactivity of solvated nanorods, colloids and dispersed systems in different liquid environments and will create new opportunities for applications in the field of geochemistry.

Structure of hematite (Fe2O3) nanorods in liquid solutions of water and ethanol

249th National Meeting (2015)

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